Why are there so few really active volcanoes and why do volcanoes become
extinct?

Tony Reay, a Geologist at Otago University, responded.

There are about 540 active volcanoes on earth and about 2500 volcanoes have
erupted within the last 10 000 years.

Early in the history of the earth,
the earth was much hotter, cooling down from a molten mass and volcanoes
were more common. Most of the interior of the earth is now solid and
volcanoes are present at the surface only above areas which can become hot
enough to melt. There are not many places within the earth where it is hot
enough to melt so there are only a few volcanoes.

Volcanoes are common in three main situations:- at the edges of continents
such as the west coast of South America, in the middle of the oceans such
as Iceland, and as strings of islands in the oceans (for example the
Hawaiian Islands). The melting to produce volcanoes takes place at a depth
of 80-100 kilometres and produces molten rock called magma. The molten rock
rises to the surface where it is stored a few kilometres below the top of
the volcano in a magma chamber. Volcanoes become extinct when the path of
the magma to the volcano at the surface is cut off and the magma chamber
cools and solidifies.

Active volcanoes in New Zealand include Ngauruhoe and White Island. Dunedin
is built on a volcano which became extinct about 10 million years ago.